The mode of interaction of hyaluronate (HA) with the surface of limb mesodermal cells changes in concert with differentiation of muscle and cartilage. The appearance of HA-binding sites on mesodermal cells corresponds with the onset of condensation of these cells, the first overt event preceding differentiation of muscle and cartilage. Past evidence supports a direct role for HA interaction with cell surface HA-binding proteins (HABPs) in the process of condensation, and, in addition, in regulation of myogenesis and in assembly of the pericellular matrix of chondrocytes. Recently, a class of HABPs (termed HABP1), that appears to be important in the above processes, has been identified in developing chick tissues, including the limb. In this proposal the following aims will be pursued towards further characterization of the molecular nature and role of HABP1: 1. HABP1 will be purified by a combination of chromatographic techniques, including immunoaffinity with already characterized monoclonal antibody to chick brain HABP1. The binding properties of purified HABP1 will be analyzed and partial amino acid sequences obtained. 2. cDNAs corresponding to HABP1 amino acid sequence, for mRNA size determination and for studies of expression during limb development. 3. Monoclonal antibody to HABP1 and defined HA oligosaccharides will be used to investigate the role of HA-HABP interactions in mesodermal condensation, regulation of myoblast fusion, and assembly of chondrocyte pericellular matrix. These studies will provide information of potential importance to our understanding of normal morphogenesis and differentiation, of developmental malformations such as occur in craniofacial anomalies and limb deformities, of repair and regenerative processes and aberrations thereof, and alterations in cell behavior during tumorigenesis.